The process for the lamination of metals currently comprises passing a metal sheet through a pair of rotating cylinders, which gives the sheet a certain thickness and hardness and, in some cases, for example in the cold lamination of flat products destined for the construction of cars and household appliances, a specific surface roughness as the geometrical surface characteristics are conveyed, in negative, onto the sheet treated.
The above-mentioned roughness parameters, and consequently the surface characteristics of lamination cylinders, are predetermined in relation to the end-use of the sheet obtained by passage through the same cylinders.
The above cylinders used for lamination must generally be periodically ground due to the deterioration undergone during the production process, and this grinding process is not always sufficient for giving the surface of the cylinder all the desired characteristics, sometimes requiring, for example in the applications mentioned above, a further surface treatment that allows a certain degree of roughness to be obtained and controlled. With respect to all the characteristics this roughness should have, it should be pointed out that said roughness is defined as a distribution of crests and craters. These craters must have more or less homogenous dimensions and must be as randomly distributed as possible.
The present known technique for treating the surface of these lamination cylinders uses different, more or less complex, technologies, of which the most widely used are sandblasting and electro-erosion, also known by experts in the field as EDT (Electro Discharge Texturing).
These treatment technologies allow a good regulation of the average roughness, but are characterized by process dangerousness and a high environmental impact, with a consequent considerable complexity in the management and disposal of the residues, in addition to the operating costs.
Sandblasting, for example, requires large-sized plants which use, for their functioning, massive turbines that are noisy and dangerous; furthermore, this process has a significant toxicity of the powders emitted by the abrasive sand, which must be purified and filtered by a specific system.
Finally, the nature of the process requires considerable maintenance due to the abrasive used which damages numerous components that cannot be adequately protected.
In addition to all of this, sandblasting does not allow a good control of the roughness and consequently the cylinders treated with this process produce a laminate which has a poor homogeneity on a roughness level.
Electro-erosion or EDT is a technology which currently offers the best results from a qualitative point of view, due to the homogeneity of the roughness obtained and total absence of traces of the machining step.
It should be noted, however, that the process is extremely dangerous due to the wide use of flammable products, such as dielectric liquids, and therefore requires the introduction of a sophisticated fire-protection system to prevent any possible sources of ignition.
The environmental impact of EDT is also considerable and even higher than that of sandblasting, as dielectric liquids are extremely toxic and must be frequently disposed of through special procedures which are also extremely costly.
A variant of the EDT process, even if less widely-used, is the EBT (Electron Beam Texturing) process, where the material is melted locally, so as to form a microcrater and deposited at the sides of the same crater.
A considerable drawback of the machines that effect this process is that the cylinder, during treatment, must be positioned in a chamber where there is a substantial forced vacuum degree, making the machine extremely expensive, difficult to maintain and ultimately unsuitable for lamination environments.
There are also other technologies that attempt to obtain the desired roughness by applying material to the surface of the cylinder rather than removing it as in the EDT technology, but the results obtained so far are not interesting for industrial applications.
Another treatment technology of cylinders currently available and capable of overcoming the considerable problems of those described above, consists of effecting an incision treatment of the surfaces of the cylinders with the use of continuous carbon dioxide laser beams or “CO2 lasers”.
The above treatment with continuous CO2 laser beams is powerful (the beams can reach power values in the order of Kw) and does not have any impact from an environmental point of view, but it is not without drawbacks, such as, for example, the fact that the conveying of the light beam towards the surface of the cylinder to be treated particularly lacks flexibility and is delicate. By operating in the “medium infrared” frequency range, in fact, (where glass is opaque and consequently optical fibers do not function), the beam is induced to incise the piece to be processed by means of mirror and lens systems and this technological limitation complicates the creation of a relative movement between the cylinder to be treated and the laser beam emitter. Furthermore, the use of a continuous high-power beam having a relatively extensive section, propagating into the air, and invisible, as it operates within the infrared frequency range, makes these machines intrinsically dangerous especially for possible ocular damage. The whole system (including the optical path) must therefore be shielded to avoid dangerous uncontrolled reflections.
The conveying limitations of the CO2 laser beam make it extremely difficult to produce surfaces without regularity, the incision sequences created with this technology tend to have passage lines on the lamination cylinders and these give the metal laminates a roughness quality that cannot be used in numerous applications, such as for example the painting of laminates suitable for being used as external parts of motor vehicles or household appliances.